Prestressing apparatus

ABSTRACT

AN APPARATUS FOR TENSIONING CABLES WHICH IS PARTICULARLY SUITABLE FOR POST-TENSIONING CABLES IN CONCRETE STRUCTURAL MEMBERS IS DISCLOSED. THE APPARATUS UTILIZES A HYDRAULIC DRIVE MEANS FOR APPLYING FORCES TO THE CABLE. A PAIR OF SPLIT WEDGES ARE UTILIZED TO SECURELY GRIP THE CABLE WHEN FORCE IS   BEING APPLIED TO THE CABLE AND ALSO WHEN THE PISTON IN THE DRIVE MEANS IS ON ITS RETURN STROKE. THE STROKE OF THE PISTON IS RELATIVELY SHORT THUS ALLOWING THE ENTIRE APPARATUS TO BE CONTAINED WITHIN A SMALL HOUSING.

United States Patent [72] Inventor Allan H. Stubbs 19113 S. Hamilton St., Gardena, Calif. 90247 [211 App]. No. 795,661 [22] Filed Jan. 31,1969 [45] Patented June 28,197]

[54] PRESTRESSING APPARATUS S C lalms, 10 Drawlng Figs.

[52] U.S. Cl. 254/29A [51] 1nt.Cl A E2lbl9/00 [50] Field ofSearch 254/29(5); 29/452; 279/28, 1 (ME); 226/(1nquired); 287/(Inquired) [56] Reterenca Cited UNITED STATES PATENTS 3,176,961 4/1965 Glass 254/29(5) 3,399,865 9/1968 Kelly 254/29(5) 3,412,511 11/1968 Dietrich 254/29(5) 3,447,784 6/1969 Launay 254/29(5) Primary ExaminerRobert C. Riordon Assistant Examiner-David R. Melton Atlomey- Spensley, Horn and Lubitz PATENTED JUN28 I971 SHEET 3 UF 3 M mwv u mw PRESTRESSING APPARATUS BACKGROUND OF THE INVENTION I. Field of the Invention The invention relates to the field of devices such as jacks for tensioning cables.

2. Prior Art In the present day highway and building construction industries post-tensioned concrete structural members are widely used. These members are typically formed on the site of the construction by setting steel cables within the concrete members. After the concrete has set, the member is placed under compression by tensioning the steel cables. The cables, once tensioned, are permanently anchored to the member.

Numerous hydraulic jacks and other devices are available for applying tension to cables. There are several disadvantages with the existing equipment, for example, the hydraulic jacks that are required to post-tension a long structural member are often too large for use on the site of the construction. Typically in a GOO-ft. beam the cable must be elongated as much as 42 inches. This would require a jack of approximately 4 feet in length, which would be unsuitable in many situations. Other devices which utilize wedges have the disadvantage of requiring continuous manual assistance since the wedges which hold or secure the cable do not readily allow the cable to slip within the wedge when necessary.

BRIEF'DESCRIPTION OF THE INVENTION An apparatus for tensioning a cable disposed through a concrete member is described. The apparatus is coupled to one end of the member and is driven by a hydraulic means comprising a piston and cylinder. The cable is axially disposed through and closely held within a duct defined by the hydraulic piston. A first wedge, mounted between the concrete member and the piston, prevents the loss of any tension in the cable by preventing the cable from moving in the direction of the concrete member. A second wedge coupled to the piston and movable with the piston securely couples the cable to the piston when the piston is applying tension to the cable. Thus, when tension is being applied to the cable, the second wedge securely holds the cable to the piston and on its return stroke the first wedge prevents the cable from sliding back into the member while the second wedges slide along the cable to regrip for the next stroke. The exterior surface of the wedges are lined with Teflon allowing them to be readily returned from their seats at the appropriate time.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the tensioning concrete member;

FIG. 2 is a side view of the tensioning apparatus with partial cutaway sections illustrating the wedges and piston utilized in the apparatus;

FIG. 3 is a view of the cable and of the relative position of the wedges when the piston is applying tension to the cable;

FIG. 4 is a sectional view of the wedge plate and housing taken through section M of FIG. 1;

FIG. 5 is a sectional view of the spring cap of the apparatus taken through section 5-5 of FIG. 1;

FIG. 6 is a sectional view of the anchor plate utilized in the concrete member of FIG. 1 taken through section 6-6 of FIG. 1;

FIG. 7 is a side view of the apparatus with a cutaway section of the piston showing the piston in the position of its furthest travel from the concrete member;

FIG. 8 is a view of the cable and the relative position of the wedges when the piston is on its return stroke;

FIG. 9 is a perspective view of a split wedge utilized to secure the cable; and,

FIG. 10 is an enlarged section of the split wedge taken along section I0-I0 of FIG. 9.

apparatus coupled to a DETAILED DESCRIPTION OF THE INVENTION Referring to FIGS. 1 and 6 the prestressing apparatus 12 is shown coupled to a concrete structural member 14. Cable 16, which is comprised of a plurality of strands 18, is disposed through the member 14 in a duct or conduit. The cable 16 is anchored at one end of member 14 by a fixed end anchor 10. Anchor 10 may be any commonly utilized fixed end anchoring means known to the art. In a typical application, after the concrete in member 14 has set, the cable 16 is tensioned by means such as apparatus 12. After the tensioning is completed the tension force is transmitted in compressionto the concrete by anchoring the stressedcable which remains as part of the structural member after the tensioning apparatus is removed.

Referring to FIGS. l, 2 and 4 housing 22 of the prestressing apparatus 12 is rigidly coupled to bearing plate 19 by means of nuts 20. The bearing plate 19 is placed within member 14.be-

fore the concrete has been poured as are the tendons and conduits. The plate 14 serves the function of transferring the loads developed within the tendon or cable 16 and thus as a compressive load to the concrete structure 14. Plate 19, a rectangular steel plate, should have sufficient area to limit the pressure applied to member 14 to a pressure less than the ultimate strength of the concrete utilized in the member 14. An aperture is disposed through bearing plate 19 so that cable 16 may freely pass through the plate.

The apparatus utilizes two split wedges to securely hold the cable; the first holds the cable when tension is being applied to it and the second holds the cable after the tension has been applied. The second wedge, holding wedge 27, is clearly illustrated in FIGS. 2, 3, 8 and 9. The wedge may be any commonly utilized split wedge having a plurality of sections as shown as sections 27a and 27b in FIG. 9. The interior of the wedge contains a plurality of teeth which are adaptable for gripping each strand 18 of cable 16 when the wedge is placed within wedge seat 52. It is preferable that the teeth of the wedge be case hardened as is shown by layer 31 in FIG. 10. The exterior surface of the wedge is preferably coated with Teflon as is illustrated by Teflon coating 45 in FIGS. 3, 8 and 9, in order that the wedge is readily released from wedge seat 52.

Wedge plate 23, comprising a rectangular steel plate, having an aperture disposed through the plate so that cable 16 may freely pass through the plate, is illustrated in FIGS. 2 and 4. Wedge seat 52, most clearly illustrated in FIGS. 3 and 8, is defined by plate 23 and is adaptable for engaging wedge 27. The seat 52 communicates with the said aperture in the wedge plate 23. Thus, when cable 16 is disposed through plate 23, wedge 27 will securely grip the cable and prevent the movement of the cable when the tension in the cable is in the direction indicated by the arrows in FIG. 8. Wedge plate 23 is held within housing 22 as is illustrated in FIG. 4 and engages bearing plate 19 through shims 21. It is within the scope of the present invention to utilize a plurality of wedges 27 each adaptable for holding one or more strands 18 of cable 16 and where wedge plate 23 contains a plurality of apertures and wedge seats 52 adaptable for receiving the said plurality of wedges 27.

Referring now to FIG. 2, stop nut 43 is adaptable for adjustably engaging threads 44 of housing 22. Disposed through the center of nut 43 is an aperture large enough to allow the free passage of wedge 27, cable 16 and spring 33. By adjusting nut 43, plate 23 may be rigidly held between shims 21 and housing 22.

Housing wedges 46 adaptable for engaging guides 48 are adjustable by means of nuts 47 within housing 22. By adjusting wedges 46 a backup barrel 50 holding spring 33 about the cable may be made to vary longitudinally with respect to the wedges 27. Spring 33 is disposed between barrel 50 and wedge 27 through the aperture in nut 43. The spring 33 provides a compressive force on the wedge to seat the wedge on the cable 16.

Referring to FIGS 2 and 7 the hydraulic drive means for the apparatus is illustrated Cylinder 26. coupled to housing 22, is adaptable for receiving and expelling hydraulic fluid through lines 40 and 41. Piston 24 is adaptable for sealingly engaging cylinder 26. The piston is illustrated in its extreme travels within cylinder 26 in FIGS. 2 and 7 Duct 28, defined by piston 24, is axially disposed through the piston. Duct 28 is adaptable for closely engaging each strand 18 of cable 16 and should have a diameter approximately equal to, but larger than the diameter of strand 18.

The force utilized to produce tension within cable 16 is provided by means of piston 24 and cylinder 26. Commonly utilized and well-known hydraulic techniques may be employed in building cylinder 26 and piston 24. The piston is so arranged that when hydraulic fluid is applied to line 40, the piston is made to move from the position shown in FIG. 2 to the position shown in FIG. 7. The piston is made to return to the position shown in FIG. 2 by applying hydraulic pressure to line 41, thereby forcing piston 24 to move towards member 14. Thus, piston 24 may be made to reciprocate within cylinder 26 by alternately applying hydraulic fluid to lines 40 and 41. It is within the scope of the present invention to utilize other reciprocating means such as pneumatic means suitable for causing piston 24 to reciprocate within cylinder 26.

Referring to FIGS. 2, 5 and 7 pulling block 37 is rigidly coupled to the piston 26. Thus, as piston 24 travels so does the pulling block 37 and the spring cap 39. Both pulling block 37 and spring cap 29 have apertures disposed through them adaptable for allowing the free passage of cable 16.

Referring to FIGS. 3 and 8, wedge seat 51 is defined by pulling block 37. The wedge seat may be similar in construction to wedge seat 52 of wedge plate 23. A wedge 29 which may be identical to wedge 27 is disposed about cable 16 within seat 51. The wedge is adaptable for preventing the cable from moving relative to pulling block 37 when the pulling block is pulling cable 16 as is illustrated in FIG. 3. Spring 37 is disposed between wedge 29 and spring cap 39 under tension so that the wedge is held within wedge seat 51 and seated to engage the cable upon the commencement of a tensioning stroke. The wedge angle, teflon interface and spring force are so constructed and arranged as to cause the wedge to unseat and release the cable upon the commencement of a return stroke as shown in FIG. 8.

Referring to FIGS. 2 and 7, the operation of the tensioning apparatus may be readily understood. First, the housing 22 is coupled to bearing plate 19 by means of bolts 20 and cable 16 is placed through the apparatus such that it enters the apparatus at wedge block 23 and leaves the apparatus at spring cap 39. Next, by adjusting nut 44, shims 22 are rigidly held between bearing plate 19 and wedge 23. Assuming first that the piston 24 is in the position shown in FIG. 2, hydraulic fluid is first applied to line 40 causing the piston to move to the position shown in FIG. 7. Referring to FIG. 3, as this occurs, pulling block 37 moves in the direction indicated by the arrows in that FIG. Since wedge 29 is held within seat 51 by spring 35, this movement of the block 37 forces the wedge to securely grip each strand 18 of cable 16. Thus, as the pulling block moves, so does cable 16 and as a result, tension is applied to the cable. Note that at the same time, the cable 16 slides within wedge 27.

On the return stroke, that is, when the piston 24 moves from the position shown in FIG. 7 to the position shown in FIG. 2, hydraulic fluid is applied to line 41. As the piston begins its movement, the cable 16 initially begins to move back into member 14. As this occurs, wedge 27 which is firmly held within seat 52 by spring 33, and (FIG. 8) is forced to securely engage and grip the cable 16. At the same time, since pulling block37is moving in the direction indicated by the arrows in FIG. 8, wedge 29 disengages seat 51. Note that the Teflon coating 45 applied to both wedges 27 and 29 allows these wedges to readily disengage their respective seats when the seat moves away from the wedge. Thus, as piston 24 continues to the position shown in FIG. 2, wedge 29 slides along the respective strand 18 of cable 16. Spring force to accomplish seating and unseating of the wedge 27 is obtained by variation of the backup barrel 50 to vary the spring pressure.

Piston 24 is made to continually reciprocate until the tension in cable 16 reaches a desired level. This level may be determined by measuring the hydraulic pressure required to move piston 24 or by measuring the elongation of cable 16.

Unlike some prior art tensioning devices, each strand 18 of cable 16 is held in a semirigid position by duct 28 of piston 24. Thus, when piston 24 is on its return stroke, the cable within the piston does not crumble. This allows wedge 29 to readily move along the cable and regrip the cable at a point closer to member 14. In addition, since spring 35 is forced to move with piston 24, the wedges 29 readily travel along the cable 16 before regripping the cable. In some prior art devices, the pulling wedge is not held in its seat by any member which is rigidly coupled to or moves with the piston. In such devices, it often becomes necessary to manually open and reset the pulling wedges so that the cable may be regripped by the wedge.

After the appropriate tension has been applied in cable 16, the hydraulic pressure is removed from piston 26 and wedge 27 is allowed to hold cable 16. Next, nut 44 is moved away from wedge plate 23 as is shown in FIG. 2. Following this, hydraulic fluid is again added to line 40 causing wedge 27 and wedge plate 23 to be moved away from bearing plate 19. At this time, a final measurement is made of the tension in cable 16 and final adjustments in the tension are made by placing an appropriate sized shim 21 between wedge plate 23 and bearing plate 19. Following this, the hydraulic pressure is removed and the housing 22 is disconnected from the bearing plate 19 thereby removing the tensioning apparatus from member 14. Typically, the cable 16 is cut and finally anchored in place using commonly known grouping techniques.

Thus, a tensioning apparatus has been disclosed which is compact in size and which may readily be manufactured utilizing standard hydraulic equipment. The Teflon-coated wedges utilized in the device readily allow a cable to be held and released at the appropriate time. The apparatus may be operated without manual assistance once it is installed on the concrete structural member.

I claim:

1. An apparatus for tensioning cable formed of a plurality of strands in a member comprising:

a first wedge means, for gripping each of said strands in said cable when said cable moves toward said member, said means coupled to said member; l

a piston defining a plurality of axially disposed ducts each receiving a respective one of said strands of said cable;

drive means, for driving said piston in reciprocating motion,

coupled to said piston and said member; and

a second wedge means, for gripping each of said strands of said cable when said cable is moved away from said member, coupled to said piston whereby when said piston is driven away from said member said second wedge means tensions the strands forming said cable, and when said piston is driven toward said member said first wedge means holds the tension in the cable.

2. The apparatus defined in claim 1 wherein said duct is of approximately the same cross-sectional shape but larger than said cable.

3. The apparatus defined in claim 1 wherein said first wedge means comprises:

a split wedge adaptable for gripping a cable, disposed about said cable; and

a wedge plate, defining a wedge seat, said seat adaptable for receiving said wedge, said plate coupled to said member.

4. The apparatus defined in claim 3 wherein said second wedge comprising:

a split wedge adaptable for gripping a cable, disposed about said cable;

a pulling block, defining a wedge seat, said seat for receiving said wedge, said block coupled to and movable with said piston at the end of said duct opposite said first wedge.

5. The apparatus defined in claim 4 wherein compression means are provided to force said second wedge into said seat and to release said wedge from said seat upon release of tension in said cable within said second wedge. 

